1. Field of the Invention
This invention relates to heater control means for refrigerated display cabinet frost control arrangements, and more particularly to heater controls directly responsive to ambient air dew point.
2. Description of the Prior Art
In stores which sell refrigerated products, such as, for example, foods to customers directly from refrigerated cabinets, the problem of displaying the refrigerated items in a continuously visible manner has been known. It is often desired to provide the items directly visible to the customer through a transparent door or window which can be opened by the customer for selecting the foods. When the substantially cooler surface of the interior face of the door suddenly comes into contact with the relatively warmer ambient air, frosting and condensation often develop on what is hoped to be the transparent door. This result follows from the fact that the temperature of the interior-facing side of the transparent door is less than the dew point of the ambient air. This problem is directly addressed, and its background discussed in greater detail, in U.S. Pat. No. 3,859,502, granted Jan. 7, 1975 to James J. Heaney.
It has long been known to coat electrically conductive material in a thin film on an optically transparent glass surface of a refrigerated cabinet door. The film is thin enough such that the glass with the film coating continues to be optically transparent. The application of electrical potential causes the film to become an electrical resistance heater, heating the transparent glass.
The usual object of heating the glass is to maintain the glass temperature above the dew point temperature of the warmer ambient air outside the cabinet. With the glass temperature maintained above the dew point, condensation and frost coating which would tend to obstruct view through the glass door is prevented.
It has also been known to provide heating wires positioned inside the frames of such refrigerated cabinet doors and possibly within mullions and other portions of the refrigerated cabinet structure for heating the frame. The heating wire would be connected to available ac electric energy to comprise an electrical resistance heater heating the adjacent frame structure. The heating wire would be electrically insulated from the frame to prevent electric shock. As with the heating of glass with the frame maintained above the dew point, condensation on the outside surfaces of the frame which might present an annoyance is prevented.
If the dew point temperature of the ambient air is high, heating the glass coated by the electrically resistive thin film and heater wires will require quite substantial amounts of electrical energy. It is often desired to reduce the amount of electrical energy applied in such applications, while retaining the benefit of a frost and condensation free, transparent refrigerated cabinet door and structure.
Suggestions have been made to selectively control power to such thin film resistance heaters and heater wires, in order to conserve electric power energy. Such a system appears to involve a sensitive device placed in abutment with the frame or mullion, or even perhaps with the transparent window itself. Prior to water condensing on the window, moisture should develop on the sensor which then, so far as is known, switches electrical energy to the heater.
Such suggestions appear to be steps in the sought or desired direction. Such systems, however, continue to apply electric heating energy at a constant rate resulting in substantial overshoot and undershoots, well known characteristics of on-off control systems. During periods of undershoot, water condensation on the window is distinctly possible. In periods of overshoot excessive electric energy is used, which may cause heating problems if not for the refrigerated cabinet interior, then for the customer shopping area outside the cabinet.